Variable Interest Entities - Additional Information (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014 VIE
Cogen Technologies Linden Venture, LP [Member]
Variable Interest Entity [Line Items]
Maximum exposure to loss	$ 105,000,000
Brooklyn Navy Yard Cogeneration Partners, LP [Member]
Variable Interest Entity [Line Items]
Maximum exposure to loss	30,000,000
Indeck Energy Services of Corinth, Inc [Member]
Variable Interest Entity [Line Items]
Maximum exposure to loss	$ 15,000,000
CECONY [Member]
Variable Interest Entity [Line Items]
Number of potential VIEs, long-term electricity purchase agreements		3